Nowadays, the amount of particulate matter in atmospheric air is commonly measured with a device called an air sampler. The air sampler draws a measured volume of air through a filter over a predetermined period of time, typically 24 hours. The weight gain of the filter, measured before and after sampling under identical conditions of temperature and humidity, divided by the volume of air passed through the filter equals the mass concentration of particulate expressed in mass per actual cubic meter of air or mass per standard cubic of air. Generally, a size selective inlet is used on an air sampler to remove particles which are greater than a certain size, for instance 10 microns or 2.5 microns, from the air, prior to particulate collection by the filter.
Most current air samplers use a single filter which must be changed after every sampling period. Newer samplers use multiple filters, each in an individual cassette, which are selected sequentially. This allows the sampler to be active for multiple individual sampling periods without operator attention. However, the multiple filter exchange procedure is time consuming, labor intensive, and may lead to errors. Typically, each filter cassette is held by an individual cassette holder which is connected to a multi-station manifold or plenum through which passes the sample stream. Each of these filter holders must be opened individually in the field, and the cassette exchanged. This has the disadvantage of requiring handling of multiple cassettes under adverse conditions which can lead to filter contamination or mis-identification.
Ideally, each filter cassette should be unambiguously identified and coordinated with the appropriate sample collection data such as time and date, total sample volume, flow rates, temperature, barometric pressure, humidity and any sampler error codes. Normally, such data is read from the air sampler and manually recorded in a notebook. In the field, the data is often collected under adverse weather conditions which makes its recording subject to human error. It would be desirable to eliminate this manual data transfer in the field, as well as the manual data taking and calculation involved in weighing the filters and reporting the final resulting mass concentrations measured by the air sampler.
In a paper entitled "A Sampling System For Reactive Species In The Western U.S.", (August 1990), Chow et al. describe a sampling system for collection of gases and particles on multiple stacked filter media, using filter holders which are apparently individually mounted in situ, to a sampler plenum. This system collects suspended particulate matter in PM.sub.2.5 (2.5 ppm) and PM.sub.10 (10 ppm) size fractions on media suitable for measurement of mass, chloride nitrate, sulfate, sodium, magnesium, potassium, and calcium ion concentrations. Gas-absorbing filters are used for the measurement of sulphur dioxide, nitrogen dioxide, nitric acid and ammonia. The versatility and utility of air samplers in measuring and monitoring atmospheric concentrations of pollutants is clearly described in this paper (the full contents of which is incorporated by reference herein).
A centrally controlled multi-station airborne particulate sampling system is described in commonly assigned International Patent Publication No. WO 94/29716 (the full contents of which is herein incorporated by reference). In this system, a paired intake and particulate filter is located in a hub unit and in each of multiple satellite units coupled to the hub unit. A vacuum pump and sampling process controller in the hub unit facilitate sequential sampling through the intake-filter pairs at the different units. This sampling system also includes a unique filter holder exchange mechanism, volume flow control, external condition activation, and sampling temperature control.
Although such known multiple filter air samplers provide significant benefits, they continue to suffer from the drawbacks described above. A need thus persists for an air sampler which can overcome these drawbacks.